Roller tappets have known advantages in certain situations since they allow increased lift velocity without increasing the body diameter. Other known advantages are increased engine breathing and durability. Conventional roller tappets comprise a central valve lifter body portion with a bifurcated end in which the roller is mounted above the central body portion. This configuration requires a lengthier tappet compared to other types of similar diameters and hence forecloses their use in many situations.
It has been suggested previously to make a more compacted roller tappet but applicant is unaware of any actual prior art examples which were developed or utilized commerically. One reason for this is attributed to the minimum longitudinal guide surface requirement in many tappets. The lifter body reciprocates in an opening in the engine block and sufficient guide surface areas are required. These guides are particularly critical in a spool-type lifter which utilizes an external recess. Referring to FIG. 1, a pair of spool-type prior art roller tappets a positioned in a portion of an internal combustion engine having a cylinder block b in cylindrical bores c in which the valve lifters a reciprocate. Each lifter a comprises a body d, the body having a lower bifurcated end e in which a roller f is rotatably mounted by a shaft. A cam shaft j is rotatably mounted in the engine and has cam lobes k which engages each roller to reciprocate the valve lifters which in turn engage pushrods l in a conventional manner. The valve lifter bodies d are spool-like and utilize a one-piece valve lifter guide or retaining clip m which is not discussed in detail. The operation of this particular type of roller tappet is set forth in U.S. Pat. No. 3,795,229. However, a recess n is required thereby limiting the length of guide annulus o. Thus the rollers f extend completely beyond the lower end of the tappets. In such or similar situations, it has not been possible heretofore to foreshorten the overall length of the roller tappet. Any attempt to recess the roller portion into the central body portion was at the expense of the guide surface o which was not acceptable. It might seem that reduction could be achieved by simply plunge milling the end of the lifter body intermediate the roller supports. However this cannot be achieved since the supports interfere with the axle of the plunge milling apparatus. While it might be possible to utilize more sophisticated milling procedures, the expense of such procedures prohibited any commercial development of such a tappet heretofore. Thus, there is a need in this art for an improved roller tappet assembly.